In matrix form, the shielding effectiveness of a conductive material depends on the amount of material used versus its aspect ratio. A material's aspect ratio is the length-to-diameter ratio of the individual conductive particles. For example, spherical conductive particles like carbon black (with an aspect ratio of unity) must be added at high levels to make plastic parts conductive. Because fibers have a higher aspect ratio, the same level of conductivity can be achieved in the plastic part using a much lower level of the conductive material in fiber form (for example, carbon fibers). This is because at the same loading level in a plastic part, fibers have a greater probability of making contact with each other than do spherical particles. With conductive fibers, this higher contact level provides more paths for an electric current to follow throughout the plastic part. Therefore, the plastic part is more conductive with the conductive material in fiber form as opposed to particle form.
Furthermore, in the context of mats that contain long conductive fibers (with aspect ratios as high as 4000) the same principle applies. The same amount of long conductive fibers in a planar configuration will be more effective at providing conductivity than they would be in a three-dimensional configuration. This is because there will be more fiber-to-fiber contact in a two-dimensional array than in a three-dimensional array for equal numbers of fibers. Therefore, plastic parts incorporating a given amount of long conductive fiber in mat form will have more conductivity than they would if the fibers were incorporated as staple, for example, loose fiber.
This analysis also applies to the planar configuration of the conductive fibers in the mat. A random distribution of conductive fibers will provide more conductivity than a directional distribution of conductive fibers because there is more fiber-to-fiber contact. Wet-laid nonwovens technology provides random fiber distribution. It follows that nonwoven mats made from wet-laid conductive fibers will have greater conductivity, everything else being equal, than woven mats with directional conductive fibers. This greater conductivity increases the shielding effectiveness of conductive nonwovens.
Thus, the wet-laid nonwovens process is the most convenient, efficient and economical way to exploit conductive material properties. Dispersion technology is used to make a nonwoven mat having long conductive fibers. By blending conductive fibers with nonconductive fibers (like polyester or glass), the conductivity of the mat can be tailored to specific market requirements. The fiber blending capabilities of the wet-laid process allow nonwoven mats to be designed with conductive fiber levels from 0.25% to 100% and basis weight from 7 to 240 g/m.sup.2. Using high-aspect-ratio conductive fiber in a nonwoven mat provides a continuous conductive matrix, even at the lowest basis weight and conductive fiber level.
Thus, a wet-laid nonwoven fiber mat having high-aspect-ratio conductive fibers, such as carbon, nickel-coated graphite (NCG) and aluminized glass, is an effective, convenient and economical way to provide ESD and EMI protection. However, such mats are disadvantageous in that they do not perform well against radiation in the low to middle megahertz frequency ranges, i.e., 10 to 200 MHz.
For example, conductive fibers consisting of carbon or nonconductive fibers coated with copper or aluminum primarily shield the electrical field and plane wave component of electromagnetic radiation. A conductive fiber content of 3-100% in a nonwoven mat also provides ESD protection. The balance of the fiber content can be polyester (e.g., polyethylene terephthalate (PET)), polypropylene, glass or other nonconductive fibers. Such mats can be incorporated in a variety of packaging materials, like plastic bags, envelopes and rigid boxes. Alternatively, conductive fibers consisting of stainless steel or nonconductive fibers coated with nickel or iron provide magnetic field attenuation.
Nonwoven mats containing 33-100% conductive fibers provide EMI shielding properties. These mats can be molded into plastic business machine housing and automobile hoods. For example, in automotive applications, a 17 g/m.sup.2, 100% NCG mat provides an average of 60 dB shielding attenuation in the 500 kHz to 120 MHz frequency range. More demanding shielding applications like architectural shielding and high-performance electronic housings require heavier weight NCG mats to provide adequate protection over a broader frequency range.
A shielded enclosure, whether it be a room or a cabinet, is only as effective as its weakest point. Although a planar material might be an excellent shielding material, it will only be one component of many in an installed system. For example, consider a room that is to be shielded. Since people must have access into and out of the room and will need environmental conditioning in the form of fresh air, heating and cooling, lighting, etc., many penetrations or openings cut into the walls will be necessary. Each opening must be shielded to prevent the entry or exit of electromagnetic radiation. Likewise, the planar conductive material used to shield the walls, floor and ceiling will need to be mated to each other as well as to the devices used to shield the openings. Hereinafter, the term "seaming" will be used to refer to the mating of sheets of the planar conductive material to each other and to the peripheral shielding devices.
In order to prevent the intrusion or escape of electromagnetic waves, enclosures are required which are electrically continuous and free of electrically transparent openings. Electromagnetic waves travel along the surface of the shielding material. Should the wave find a hole or gap having a dimension greater than some critical value, the wave can pass through and thereby radiate into or leak out of the protected area. The leakage of electromagnetic radiation through any opening depends on two factors: the largest dimension D of the opening and the wavelength w of the radiation. When w&lt;2D, the radiation will pass through the opening freely. When this happens, the room is said to leak and the shielding will be ineffective in protecting the equipment in the room against EMI. Any frequency for which w&gt;2D will be unable to penetrate the opening.
This principle applies to the nonwoven fabric as well as to seams. If the dimension of the interfiber spaces of the nonwoven mat is D, then all wavelengths greater than 2 D will be cut off. Similarly, any opening in the seam must have a dimension less than one-half of the shortest wavelength to be shielded against.
One of the most difficult problems with conventional planar shielding media such as metal foil, wire mesh and sheet (gauge) metal has been how to effect EM-radiation-tight seams that will not open when the structure is stressed. A good seaming technique is to weld or solder all seams, but this is expensive, time-consuming and not always practical. Gasketing and compression bolting is often used, but these can loosen with time and therefore must be accessible to enable periodic retorquing. Furthermore, if the bolts are overtorqued, bulging of the panel or sheet can arise, resulting in a gap of sufficient size to enable passage of EM waves therethrough. Unless painstaking attention is paid to seaming construction, the shielding effectiveness of such materials degrades at higher frequencies. Special copper seaming tapes with pressure-sensitive adhesive are available but are prone to releasing off of the substrate wall over time.
Foils have traditionally been used to tape across seams. Because foils are impermeable, the adhesive used to adhere the tape must be conductive or it will form an insulating layer leading to electrical discontinuity between conductive surfaces to be joined. Conductive adhesive is expensive and to eliminate their use with foils, a dimple pattern is embossed into the foil. When carefully applied with appropriate pressure the dimple pushes through the adhesive to provide direct contact with the conductive surfaces to be seamed. These foils are difficult to work with and must be installed carefully to avoid wrinkles or gaps that could introduce electrically discontinuous areas and, therefore, EM radiation leakage.
Shielding tapes based solely on metal foil are further disadvantageous because of the poor mechanical performance of foil. Metal foil is susceptible to folding and creasing, which creates a waveguide opportunity for electromagnetic radiation.
In contrast, conductive nonwoven fabric material is easily seamed, so that degradation of the shielding effectiveness for higher-frequency radiation does not occur. However, such nonwoven systems are not entirely satisfactory in that conductive nonwoven materials do not perform well in the middle frequency ranges.